1. Field of the Invention
The present invention relates to a flexible printed circuit board for establishing the electrical connection in, for example, an electronic apparatus. More particularly, the present invention relates to a flexible printed circuit board which incorporates a conductor, such as copper foil which is directly applied with polyimide resin, and which has an insulating protective layer made of the polyimide resin and formed thereon.
2. Description of the Related Art
To reduce the size and cost of a so-called portable electric product, such as a portable recording/reproducing apparatus, the flexible printed circuit board is usually employed. The flexible printed circuit board establishes the connection among the electric circuits to one another, the flexible printed circuit board being a relatively low cost board which permits space saving.
In the industrial field of the flexible printed circuit boards, there are requirements for reducing the sizes of the electronic apparatuses and permitting a multiplicity of functions to be provided. Therefore, techniques have rapidly been progressed, the techniques including raising of the mounting density of the circuits, surface mounting which uses wire bonding, direct mounting of a semiconductor chip and packaging. Therefore, the shape of the flexible printed circuit board becomes more complicated. Moreover, small circuits for use in the dense mounting process have been required.
The conventional flexible printed circuit board incorporates a circuit formed by etching a flexible printed board which is a base board. Then, an insulating protective layer (a cover layer) is formed to protect the circuit.
The flexible printed circuit board, which is the base board, for use in the flexible printed circuit board usually incorporates polyimide films because of their satisfactory flexibility and heat resistance. The flexible printed board is exemplified by a triple-layer flexible printed board constituted by bonding a polyimide film and a copper foil, which is a conductor, to each other through a heat-resisting adhesive.
However, the foregoing triple-layer flexible printed board suffers from unsatisfactory heat resistance of the adhesive. Thus, the adhesive is easily deformed owning to heat. Therefore, heat hysteresis, such as heat crimping, which is performed when a bonding process is performed by using the adhesive causes warp or curl of the board to occur. As a result, there arises problems in that a small circuit pattern cannot easily be formed and that surface mounting which uses wire bonding cannot easily be performed.
To overcome the problems experienced with the triple-layer flexible printed board, a double-layer flexible printed board 53 has been suggested and put into practical use. As shown in FIG. 1, no adhesive is employed and polyamic acid, which is a precursor for the polyimide, is directly applied to the surface of metal foil 51 made of copper or the like. Then, the polyamic acid is dried, and then it is turned into an imide so that a polyimide resin layer 52 is laminated so that the double-layer flexible printed board 53 is manufactured. A flexible printed circuit board 50 incorporating the double-layer flexible printed board 53, as shown in FIG. 2, has a circuit formed thereon. Moreover, an insulating protective layer 54 is formed on the circuit. In FIG. 2, a portion of the metal foil 51 which is not coated with the insulating protective layer 54 and which is exposed to the outside is a portion which will be formed into a terminal.
Also the double-layer flexible printed board 53 must be subjected to a high-temperature process in which the polyamic acid applied to the surface of the metal foil 51 is turned into an imide. Although the polyimide resin layer 52 having heat resistance superior to that of the foregoing adhesive is employed, the difference between the coefficient of linear thermal expansion of the metal foil 51 and that of the polyimide resin layer 52 causes the following difference. That is, the thermal contraction ratio becomes different between the metal foil 51 and the polyimide resin layer 52 after the temperature has been lowered to room temperature. Thus, a curl is undesirably formed. If the double-layer flexible printed board 53 is curled, an accuracy of the interval between conductors of the circuit deteriorates after the etching process has been performed. What is worse, mounting of elements cannot easily be performed.
Therefore, a method of preventing the curl of the double-layer flexible printed board has been suggested. The method has the step of specifying the chemical structure of the polyimide to be formed. To minimize the coefficient of linear thermal expansion of the polyimide resin, a method has been suggested with which the structure of the polyamic acid, which is the precursor, is specified.
However, either of the methods of preventing the curl of the double-layer flexible printed board cannot easily completely prevent the curl. When the circuit has been formed by etching the metal foil, the curl cannot be removed.
The foregoing conventional methods have been adapted to the double-layer flexible printed board 53, which is the base board on which the insulating protective layer 54 has not been formed. Therefore, no investigation has been made about the flexible printed circuit board 50 which incorporates the double-layer flexible printed board 53 and the insulating protective layer 54 for the circuit which are integrated with each other.
The industrial field of the flexible printed circuit boards have recently be required to be technically progressed to realize, for example, dense mounting. Since the semiconductor chip is mounted by using soldering, the required performance of the flexible printed circuit board can satisfactorily be realized when the flexible printed board and the insulating protective layer for the circuit are considered as one device. Therefore, prevention of the curl of the double-layer flexible printed board 53 is insufficient to realize the performance. As a result, a state of the flexible printed circuit board 50 incorporating the insulating protective layer 54 mounted integrally must be investigated.
The insulating protective layer 54 (the cover layer) for use in the flexible printed circuit board 50 must have heat resistance. Therefore, the insulating protective layer 54 is usually made of the polyimide material. The insulating protective layer 54 can be formed by a printing method or a film method.
The printing method employs a step similar to a step which is performed with a hard printing plate to print resist ink by using a silk screen. Since the resist ink is mainly composed of epoxy resin, the foregoing method has a problem of unsatisfactory flexibility. If the resist is not selected in sufficient consideration of the thermal expansion ratio of the resist, the flexible printed circuit board undesirably encounters a curl. Since solvent having a high polarity and contained in the polyimide resist ink absorbs water in the atmosphere, satisfactory printing workability cannot be realized. Thus, there arises a problem in that the thicknesses of the films cannot easily be administered.
On the other hand, the film method has the steps of forming apertures in portions corresponding to the land portions and terminal portions by using a die or a punch. Then, a polyimide film applied with an adhesive and a flexible printed board having a circuit formed thereon are bonded to each other by heat crimping or the like. Thus, a flexible printed circuit board is manufactured.
The foregoing method, however, encounters difficulty in forming fine land portions and terminal portions though the accuracy of the die or the like is improved. Moreover, the method has a problem in that leakage of the adhesive contaminates the fine circuit. When the flexible printed circuit board is mounted on the semiconductor chip or the like, the flexible printed circuit board must have flatness. When the insulating protective layer is formed, dispersion of the thicknesses of the insulating protective layers causes partial contraction or a curl to occur. As a result, the flatness deteriorates and undesirable contraction takes place between the conductors. Recently, a circuit has been required which has a structure that the insulation between conductors of a fine circuit in the terminal portion or the like, that is, insulation between adjacent conductors is realized by embedding an insulating protective layer between the conductors. In the foregoing case, severe dimension stability between the conductors is required. The conventional technique cannot meet the foregoing requirement.
As described above, the flexible printed circuit board 50 has problems when the insulating protective layer 54 is formed. The problems include curl caused from incorrect selection of the resist, separation of the resist, resistance against breakage, leakage of the adhesive, insufficient bonding force and deterioration in the dimension accuracy.
In view of the foregoing, an object of the present invention is to provide a flexible printed circuit board which is capable of preventing a curl, exhibiting excellent dimension stability and having high performance and which is adaptable to a fine and dense circuit required to reduce the size and increase the function of an electronic apparatus.
To achieve the above-mentioned object, according to one aspect of the present invention, there is provided a flexible printed circuit board comprising: a conductor patterned to correspond to a circuit; a first polyimide-resin layer formed on either surface of the conductor and capable of supporting the conductor; and a second polyimide-resin layer formed on another surface of the conductor and capable of covering and protecting the circuit, wherein the difference between the coefficient of linear thermal expansion of the first polyimide-resin layer and the coefficient of linear thermal expansion of the second polyimide-resin layer is 3xc3x9710xe2x88x926/K or smaller.
The flexible printed circuit board according to the present invention incorporates the first polyimide-resin layer and the second polyimide-resin layer which are formed by applying polyamic acid, which is a precursor for the polyimide resin, and by turning the polyamic acid into an imide.
As described above, the flexible printed circuit board according to the present invention has the structure that the difference in the coefficient of linear thermal expansion between the first polyimide-resin layer and the second polyimide-resin layer is optimized. Therefore, the thermal expansion of the two polyimide resin layer formed across the conductor are made to be the same as much as possible. As a result, the flexible printed circuit board according to the present invention is able to prevent a curl and maintain satisfactory flatness if the flexible printed circuit board is subjected to a process, such as a heating process.
Moreover, the flexible printed circuit board according to the present invention incorporates the second polyimide-resin layer which is formed by applying the polyamic acid. Therefore, the flexible printed circuit board according to the present invention is free from problems, such as curl caused from incorrect selection of the resist, separation of the resist, poor resistance against breakage, leakage of the adhesive, insufficient bonding force and deterioration in the dimension accuracy. Therefore, occurrence of a curl can efficiently be prevented and satisfactory flatness can be maintained.